1. Field of the Invention
This invention relates to an electric insulator being capable of insulating between high voltage stem pins in a cathode ray tube used for electric equipment provided with high voltage electrodes, which electric equipment particularly includes color television picture tubes and monitors of image screens and computers, and for other electrodes being subjected to a high voltage, and relates to an electric insulation device used thereof, and further relates to a method of electrical insulation using the electric insulator.
2. Description of the Related Art
The cathode ray tube described above is a typical example of electric equipment provided with high voltage electrodes. The high voltage inter-stem pin electric insulator in a cathode ray tube of the prior art and the method for producing a cathode ray tube using the insulator are described below referring to FIG. 1 and FIG. 2.
FIG. 1 illustrates the method of electric insulation between stem pins including a high voltage stem pin of the prior art. FIG. 1A shows a stem base, and FIG. 1B shows a neck of the cathode ray tube. FIG. 2 illustrates the neck of the cathode ray tube structured in accordance with the method of electrical insulation illustrated in FIG. 1. FIG. 2A gives a view of the neck, and FIG. 2B gives the cross sectional view along the A--A line in FIG. 2A.
In FIG. 1B, the reference number 1 designates the neck of a cathode ray tube which is installed in a color television picture tube or a monitor for an image screen or computer. The neck accepts an electron gun being mounted on a stem 2, which is not shown, by insertion therein, and the stem 2 is provided with more than one stud electrode, stem pins 3A, 3B, 3C, etc. The glass periphery of the stem 2 and the glass edge periphery of the neck 1 are welded together to integrate them. The assembled cathode is evacuated through the tip 4 of the exhaust pipe formed at the center of the stem 2, then the tip 4 is sealed. The stem pins 3A, 3B, 3C, etc. are studded along a specified pin pitch circle.
In FIG. 1A, the reference number 10 designates a stem base. The stem base 10 consists of a bottom disc plate 11, a tip acceptor 12 which is formed at the center of the bottom disc plate 11 and protects the tip 4, an acceptor for high voltage stem pin 13 in a fan-shape to receive a high voltage stem pin 3B and act also as a part of the side wall of the tip acceptor 12, a skirt 14 which covers the edge periphery of the neck 1 formed at the periphery of the bottom plate 11 and which prevents sagging of the paste electric insulator 5, and more than one throughhole, 15A, 15B, 15C, etc. aligning to the stem pins 3A, 3B, 3B, etc. formed at the periphery of the bottom plate 11.
A common material for the stem base 10 is a resin such as polycarbonate which has excellent insulations properties. The dielectric breakdown voltage of polycarbonate resin is 30 KV/mm.
A conventional method to electrically insulate between the stem pins 3A, 3B, 3C, etc. being studded onto the stem 2 on the neck 1 of the cathode ray tube is to evacuate the cathode ray tube, to seal the tip 4, then, as illustrated in FIG. 1B, to insert the nozzle of a tube 6 filled with paste electric insulator 5 between the stem pins 3A, 3B, 3C, etc. of the stem 2, and to squeeze out a small amount of the electric insulator 5 from the tube 6 using fingers 7.
The stem base 10 shown in FIG. 1A is placed on the applied electric insulator 5 in a manner that the tip acceptor 12 accepts the tip 4 and that the acceptor 13 for high voltage stem pin 3B accepts the high voltage stem pin 3B and that each of the throughholes 15A, 15B, 15C, etc. accepts a corresponding stem pin 3A, 3B, 3C, etc. under a slight compression. Then, the configuration becomes the one illustrated in FIG. 2. The structure assures the electric insulation between the stem pins, 3A, 3B, 3C, etc.
As realized in the present color television picture tubes and monitors for image screens and computers, the cathode ray tubes for this equipments are requested to have high brightness and high resolution. To cope with the ever increasing demand, a high voltage is applied not only to the anode of the cathode ray tube but also to the electrode of the electron gun in the cathode ray tube. In particular, a focus electrode is unavoidably applied with a high voltage for improving the focusing characteristic.
The outside diameter of the neck 1 receiving the electron gun has an upper limit from the standpoint of saving the deflection power of the electron gun. Consequently, the area of stem 2 is limited. Since such a limited area of the stem 2 receives the stem pins 3A, 3B, 3C, etc., which receive various levels of voltage, the spacing of these stem pins 3A, 3B, 3C, etc. becomes close.
For example, a color cathode ray tube for monitoring a graphic display made by SONY CORP. has the pin pitch circle diameter of 15.24 mm, and the number of studded pins is 14 with the pin spacing of 3.42 mm.
Among these stem pins, 3A, 3B, 3C, etc., the stem pin 3A, for example, is called the G1 pin which is applied with a voltage of 0 to several volts, and the stem pin 3B is the one for focus electrode which is applied with maximum 9800 volts, and the stem pin 3C is the one to supply voltage to the convergence electrode and is applied with 0 to 2000 volts.
Accordingly, there exists a maximum difference of 9800 volts between the stem pin 3A and the stem pin 3B, and a maximum of 8750 volts between the stem pin 3B and the stem pin 3C. For a new cathode ray tube, or if the surface of the glass stem 2 is clean, the glass dielectric breakdown voltage (the lead along the stem glass surface) between the stem pins 3A and 3B and between the stem pins 3B and 3C is not problem. However, a temperature rise or deposition of dust or dirt caused by static charge degrades the dielectric breakdown voltage during operation, or an excess voltage over the dielectric breakdown voltage is often applied to the stem pins, so the dielectric breakdown voltage is not necessarily secured during operation.
To assure the necessary dielectric breakdown voltage, conventional technology uses a paste silicone resin of a room-temperature-curing type such as KE03490 made by Shin-Etsu Chemical Co. Ltd. and SE-9168 made by Toray Dow Corning Co., Ltd. That type of resin is hereinafter referred to simply as "RTV resin".
The RTV resin has basic electric characteristics, but it lacks flowability because of its paste property. As illustrated in FIG. 1B, the RTV resin 5 is squeezed out from the tube 6 between the stem pins before mounting the stem base 10 to the stem 2, then the stem base 10 is attached. Usually, the application of RTV resin 5 is manually carried out.
However, the RTV resin raises several problems in that it likely develops bubbles during application. A solvent existing in resin foams at a temperature range of from 80 to 120.degree. C. under the heat generated during the aging stage for activating the cathode of the electron gun. Bubbles also occur during the insertion and withdrawal of connector of an instrument employed several times for inspection in the period between application of the RTV resin and the complete curing, (normally 7 days). Bubbles interfere with the perfect functioning of the electric characteristics of the resin and degrade the dielectric breakdown voltage and, in the worst case, discharge occurs between the causing damage to stem pins to damage the cathode ray tube.